One of the newer and growing VOIP companies is called Packet 8. VOIP (Voice over IP) using your existing High Speed Internet connection. We recommend a Broadband connection, as sometimes you will notice some noise on a DSL line, especially if your are downloading any large files over the internet while talking on the phone. Most providers you will be able to use your existing phones, but some are will not. We have found the Packet 8 to be very reliable and have recently made the switch. You can see there information here Packet 8
A great technology resource to help compare and get questions answered is :
Packet8 is an affordable and easy-to-use broadband telephone and videophone service. Designed with both residential and business customers in mind, Packet8 allows anyone with broadband (high-speed) Internet access to use their regular phone to make UNLIMITED calls to anywhere in the U.S. and Canada for as little as $19.99 per month. Packet8 subscribers with videophones can make video calls for as little as $19.99 per month. All Packet8 subscribers get world wide UNLIMITED calling to other Packet8 subscribers at no extra charge.